Bipolar carrier injection material and preparation method and application thereof

Abstract

The invention discloses a bipolar carrier injection material and a preparation method and application thereof, and relates to the technical field of semiconductor materials. The material is generated after hydrothermal reaction of urea and transition metal salt, and can be used for preparing a hole injection layer and / or an electron injection layer of an organic light-emitting diode device. The bipolar carrier injection material which is good in film-forming property and stability and can be treated by a solution is prepared by a simple hydrothermal method based on an inorganic material with low cost, so that the purpose that the same material can be used as a hole injection layer and an electron injection layer is achieved, the complexity and the production cost of generation equipment are reduced, and the production efficiency is improved. And an effective and feasible new thought is provided for researching an efficient ultraviolet OLED bipolar carrier injection material.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor materials technology, and in particular relates to a bipolar carrier injection material, its preparation method, and its application. Background Technology

[0002] Organic light-emitting diodes (OLEDs), as a new generation of flat panel display and solid-state lighting technology, have become a research hotspot in the optoelectronic field due to their significant advantages such as self-emission, high contrast, wide viewing angle, fast response speed, and flexibility. Among them, ultraviolet OLEDs (UV-OLEDs) have received widespread attention from academia and industry in recent years due to their unique application value in photolithography, biosensing, high-density information storage, excitation light sources, and sterilization. However, due to the intrinsic characteristics of the wide bandgap of ultraviolet emitting materials, their highest occupied molecular orbital (HOMO) energy level is extremely deep, resulting in severe challenges for carrier injection.

[0003] First, the work function of conventional transparent anodes (such as ITO) is significantly mismatched with the HOMO energy levels of ultraviolet luminescent materials, forming a huge hole injection barrier. This makes it difficult for holes to be effectively injected into the luminescent layer, resulting in a severe imbalance of charge carriers inside the device and low luminous efficiency. Second, on the electron injection side, to overcome the energy level barrier, existing technologies typically rely on reactive metals such as lithium, cesium, or their compounds as electron injection materials. These materials are chemically extremely unstable and sensitive to water and oxygen, which not only increases the difficulty of device packaging and the risk of failure, but also makes their vapor deposition process complex and difficult to control precisely, severely restricting the stability and large-scale fabrication of the devices.

[0004] More importantly, the simultaneous existence of the challenges of hole injection and electron injection, coupled with their drastically different solutions (high hole injection capability required on the anode side, and low work function electron injection capability required on the cathode side), means that very few single materials can simultaneously possess both highly efficient hole and electron injection performance (i.e., bipolar injection characteristics). In existing UV OLED device fabrication processes, different materials are needed to fabricate the hole injection layer and electron injection layer separately, requiring at least two independent evaporation sources during vacuum thermal evaporation. This significantly increases equipment complexity and production costs, reduces production efficiency, and limits the large-scale application of UV OLEDs in flexible electronics and multifunctional integrated devices.

[0005] Therefore, developing a novel bipolar injection material with a suitable energy level structure, high thermal stability, and excellent film-forming properties, which can effectively bridge the hole injection barrier between the ITO anode and the deep-level ultraviolet light-emitting material, and replace unstable active metals to achieve efficient electron injection, thereby enabling a single material to have bidirectional injection capabilities, reducing equipment complexity, and lowering production costs, is a key technical problem that urgently needs to be solved in the current ultraviolet OLED technology field. Summary of the Invention

[0006] The purpose of this invention is to provide a bipolar carrier injection material, its preparation method, and its applications, aiming to at least partially solve the technical problems in the prior art. To this end, this disclosure provides the following technical solution:

[0007] In a first aspect, the present invention provides a method for preparing a bipolar carrier injection material, which is generated by hydrothermal reaction of urea and a transition metal salt; the transition metal salt contains at least one transition metal selected from nickel (Ni), cobalt (Co), and zinc (Zn); the hydrothermal reaction is carried out at a temperature of 175-185°C for a time of 8-12 hours.

[0008] In one or more embodiments, the transition metal salt is selected from at least one of nickel nitrate, cobalt nitrate, and zinc nitrate.

[0009] In one or more embodiments, the molar ratio of urea to transition metal salt is 1:(0.8–1.5).

[0010] In one or more embodiments, the preparation method further includes purification, the process of which involves cooling the hydrothermal reaction solution to room temperature and centrifuging to collect the supernatant. Further, the centrifugation conditions are: 10000–15000 rpm for 5–10 minutes.

[0011] In one or more embodiments, the preparation method further includes a coating process: spin-coating the supernatant onto a substrate, followed by annealing to obtain a thin film. Further, the spin-coating speed is 3000–3500 rpm for 60–80 seconds; the annealing temperature is 130–220°C for 10–20 minutes. Further, the substrate is ITO glass, and the substrate surface is cleaned before spin-coating by sequentially performing ultrasonic cleaning with acetone (purity not less than 99%), isopropanol (purity not less than 99%), and deionized water for 10–20 minutes, followed by ultraviolet ozone treatment (UV-O3) for 10–20 minutes.

[0012] Secondly, the present invention also provides a bipolar carrier injection material prepared by the above-described preparation method.

[0013] Thirdly, the present invention also provides the application of the above-mentioned bipolar carrier injection material in the fabrication of organic light-emitting diode devices.

[0014] Fourthly, the present invention also provides an organic light-emitting diode device, comprising an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer, and a cathode. The hole injection layer and / or the electron injection layer are formed by coating the aforementioned bipolar carrier injection material.

[0015] Compared with the prior art, the present invention can achieve at least the following beneficial effects:

[0016] 1. This invention utilizes low-cost inorganic materials to prepare thin films via a simple and controllable hydrothermal method combined with solution spin coating. The resulting thin films have a smooth and dense surface, enabling uniform deposition on flexible substrates. Furthermore, the materials exhibit excellent thermal stability, withstanding subsequent high-temperature annealing without decomposition. This solution-processable characteristic makes them particularly suitable for large-area coating and flexible substrates, providing a new approach for researching high-efficiency bipolar carrier injection materials for ultraviolet OLEDs and offering a new feasible solution to meet the industrialization needs of flexible and multifunctional devices.

[0017] 2. This invention utilizes the coordination structure formed by urea and transition metals (nickel, cobalt, zinc) to cleverly control the electron cloud distribution and energy level positions of the material. The introduction of transition metal ions not only enhances the electron injection capability through O→M coordination bonds, replacing water- and oxygen-sensitive reactive metals such as lithium and cesium, significantly improving the environmental stability of the device; simultaneously, the conjugated rearrangement of the transition metal base unit with a wide valence band and urea molecules effectively reduces the hole injection barrier, bridging the energy gap between the ITO anode and the deep HOMO energy level emitting layer. Experimental data show that the material achieves a maximum external quantum efficiency (EQE) of 1.73% when used as a hole injection layer and 1.19% when used as an electron injection layer, successfully solving the core problem of carrier injection imbalance in ultraviolet OLEDs.

[0018] 3. The material provided by this invention possesses excellent intrinsic bipolar properties. The same material can serve as both a highly efficient hole injection layer and a stable electron injection layer. During device manufacturing, only one evaporation source or solution processing unit is needed to prepare the bipolar injection layer, significantly saving expensive vacuum coating equipment resources and substantially reducing production time and manufacturing costs. This provides a highly competitive process solution for the large-scale industrialization of ultraviolet OLEDs. Attached Figure Description

[0019] Figure 1 This diagram illustrates the fabrication process of bipolar carrier injection materials and their use as hole injection layer (HIL) and electron injection layer (EIL) in OLEDs.

[0020] Figure 2 Atomic force microscopy (AFM) images (scanning area 3.0 μm × 3.0 μm): (a) ITO anode, (b) ITO / Urea, (c) ITO / Urea + Ni, (d) ITO / Urea + Co, (e) ITO / Urea + Zn.

[0021] Figure 3 Scanning electron microscope (SEM) images of (a) Urea+Ni, (b) Urea+Co and (c) Urea+Zn, and corresponding energy-dispersive X-ray spectral (EDS) elemental distribution maps.

[0022] Figure 4 X-ray photoelectron spectroscopy (XPS) spectra of Urea, Urea+Ni, Urea+Co and Urea+Zn thin films prepared by spin coating: (a) C 1s, (b) O 1s, (c) N 1s, (d) Ni 2p, (e) Co 2p, (f) Zn 2p.

[0023] Figure 5 UV-Vis absorption spectra of Urea, Urea+Ni, Urea+Co, and Urea+Zn thin films prepared by spin coating.

[0024] Figure 6 The relevant curves or spectra of OLEDs (corresponding to devices EU, EN, EC, and EZ) with Urea, Urea+Ni, Urea+Co, and Urea+Zn as electron injection layers (EIL) are shown. Among them: (a) current density-voltage (J–V) characteristic curve, (b) irradiance curve, (c) external quantum efficiency (EQE) curve, and (d) normalized electroluminescence (EL) spectrum.

[0025] Figure 7 Electroluminescence (EL) spectra of device (a) EU, device (b) EN, device (c) EC, and device (d) EZ at different voltages.

[0026] Figure 8 The relevant curves or spectra of OLEDs (corresponding to devices HN, HC, and HZ) with Urea+Ni, Urea+Co, and Urea+Zn as hole injection layers (HIL) are shown. Among them: (a) current density-voltage (J–V) characteristic curve, (b) irradiance curve, (c) external quantum efficiency (EQE) curve, and (d) normalized electroluminescence (EL) spectrum.

[0027] Figure 9 Electroluminescence (EL) spectra of device (a) HN, device (b) HC, and device (c) HZ at different voltages.

[0028] Figure 10 The following are the characteristic transition curves of (a) current-voltage (I–V), (b) impedance-voltage (Z–V), and (c) phase angle-voltage (Φ–V) of an electron-single-carrier device (EOD).

[0029] Figure 11 The following are the characteristic transition curves of (a) current-voltage (I–V), (b) impedance-voltage (Z–V), and (c) phase angle-voltage (Φ–V) of a hole-single-carrier device (HOD). Detailed Implementation

[0030] The present invention will now be described in detail with reference to exemplary embodiments or experimental examples shown in the accompanying drawings. However, it should be understood that the present invention can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided herein to make the disclosure of the present invention more complete and to fully convey the inventive concept to those skilled in the art.

[0031] Example 1

[0032] refer to Figure 1 OLED devices consist of an anode, a hole injection layer (HIL), a hole transport layer (HTL), an emissive layer (EML), an electron transport layer (ETL), an electron injection layer (EIL), and a cathode. In this embodiment of the invention, a precursor solution is first prepared, and then deposited onto ITO glass to form a hole injection layer or an electron injection layer.

[0033] (1) Preparation of precursor solution

[0034] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.485g of nickel nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 180°C for 10h. After cooling to room temperature, centrifuge at 12000rpm for 8 minutes. Then take the supernatant to obtain the precursor solution (Urea+Ni).

[0035] (2) OLED device fabrication

[0036] OLED devices are all fabricated on ITO coated glass. The ITO glass is first ultrasonically cleaned for 15 minutes with 99.5% acetone, 99.7% isopropanol and deionized water, and then pretreated with UV-O3 for 15 minutes to remove impurities on the ITO glass surface and improve the work function.

[0037] The electron-injected layer (EIL) was prepared by spin-coating a precursor solution (Urea + Ni) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 200°C for 15 min. Subsequently, the ITO glass coated with the EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. - 4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) for HTL, MoO3 for HIL, and Al for anode were sequentially deposited to obtain an OLED device with the following structure:

[0038] Device EN: ITO / Urea+Ni / Bphen (25nm) / TAZ (35nm) / CBP (70nm) / MoO3 (5nm) / Al (110nm).

[0039] HIL was prepared by spin-coating a precursor solution (Urea + Ni) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 150°C for 15 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially deposited to obtain an OLED device with the following structure:

[0040] Device HN: ITO / Urea+Ni / CBP (32nm) / TAZ (35nm) / Bphen (170nm) / LiF (2nm) / Al (110nm).

[0041] Example 2

[0042] (1) Preparation of precursor solution

[0043] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.485g of cobalt nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 180°C for 10 hours. After cooling to room temperature, centrifuge at 12000rpm for 8 minutes. Then take the supernatant to obtain the precursor solution (Urea+Co).

[0044] (2) OLED device fabrication

[0045] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0046] EIL was prepared by spin-coating a precursor solution (Urea + Co) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 200°C for 15 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) for HTL, MoO3 for HIL, and Al for anode were sequentially deposited to obtain an OLED device with the following structure:

[0047] Device EC: ITO / Urea+Co / Bphen (25nm) / TAZ (35nm) / CBP (70nm) / MoO3 (5nm) / Al (110nm).

[0048] HIL was prepared by spin-coating a precursor solution (Urea + Co) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 150°C for 15 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially deposited to obtain an OLED device with the following structure:

[0049] Device HC: ITO / Urea+Co / CBP (32nm) / TAZ (35nm) / Bphen (170nm) / LiF (2nm) / Al (110nm).

[0050] Example 3

[0051] (1) Preparation of precursor solution

[0052] Dissolve 0.09g of urea in 15ml of deionized water to prepare a pure urea solution. Mix 0.495g of zinc nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 180°C for 10 hours. After cooling to room temperature, centrifuge at 12000rpm for 8 minutes. Then take the supernatant to obtain the precursor solution (Urea + Zn).

[0053] (2) OLED device fabrication

[0054] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0055] EIL was prepared by spin-coating a precursor solution (Urea + Zn) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 200°C for 15 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) for HTL, MoO3 for HIL, and Al for anode were sequentially deposited to obtain an OLED device with the following structure:

[0056] Device EZ: ITO / Urea+Zn / Bphen (25nm) / TAZ (35nm) / CBP (70nm) / MoO3 (5nm) / Al (110nm).

[0057] HIL was prepared by spin-coating a precursor solution (Urea + Zn) onto ITO glass at 3200 rpm for 70 s, followed by annealing at 150°C for 15 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially deposited to obtain an OLED device with the following structure:

[0058] Device HZ: ITO / Urea+Zn / CBP (32nm) / TAZ (35nm) / Bphen (170nm) / LiF (2nm) / Al (110nm).

[0059] Example 4

[0060] (1) Preparation of precursor solution

[0061] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.36g of nickel nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 175°C for 10 hours. After cooling to room temperature, centrifuge at 15000rpm for 5 minutes and then take the supernatant to obtain the precursor solution.

[0062] (2) OLED device fabrication

[0063] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0064] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3200 rpm for 80 s, followed by annealing at 180°C for 15 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 80 s. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0065] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3500 rpm for 60 s, followed by annealing at 150°C for 20 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0066] Example 5

[0067] (1) Preparation of precursor solution

[0068] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.64g of nickel nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 180°C for 12 hours. After cooling to room temperature, centrifuge at 10,000rpm for 8 minutes and then take the supernatant to obtain the precursor solution.

[0069] (2) OLED device fabrication

[0070] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0071] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3500 rpm for 60 s, followed by annealing at 200°C for 20 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0072] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3000 rpm for 70 s, followed by annealing at 170°C for 10 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 70 s. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0073] Example 6

[0074] (1) Preparation of precursor solution

[0075] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.36g of cobalt nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 185°C for 8 hours. After cooling to room temperature, centrifuge at 12000rpm for 10 minutes. Then, take the supernatant to obtain the precursor solution.

[0076] (2) OLED device fabrication

[0077] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0078] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3000 rpm for 70 s, followed by annealing at 220°C for 10 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0079] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3200 rpm for 80 s, followed by annealing at 130°C for 15 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 80 s. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0080] Example 7

[0081] (1) Preparation of precursor solution

[0082] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.64g of cobalt nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 185°C for 8 hours. After cooling to room temperature, centrifuge at 15000rpm for 5 minutes. Then take the supernatant to obtain the precursor solution.

[0083] (2) OLED device fabrication

[0084] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0085] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3500 rpm for 60 s, followed by annealing at 220°C for 10 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0086] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3500 rpm for 60 s, followed by annealing at 170°C for 10 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 60 s. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0087] Example 8

[0088] (1) Preparation of precursor solution

[0089] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.37g of zinc nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 175°C for 12 hours. After cooling to room temperature, centrifuge at 10000rpm for 10 minutes and then take the supernatant to obtain the precursor solution.

[0090] (2) OLED device fabrication

[0091] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0092] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3000 rpm for 80 s, followed by annealing at 180°C for 20 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 20 min. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0093] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3000 rpm for 80 s, followed by annealing at 130°C for 20 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 20 min. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0094] Example 9

[0095] (1) Preparation of precursor solution

[0096] Dissolve 0.09g of urea in 15ml of deionized water to prepare a urea solution. Mix 0.66g of zinc nitrate hexahydrate with the urea solution, sonicate, and place in a high-pressure reactor. Perform a hydrothermal reaction at 180°C for 10 hours. After cooling to room temperature, centrifuge at 15000rpm for 10 minutes. Then, take the supernatant to obtain the precursor solution.

[0097] (2) OLED device fabrication

[0098] The ITO glass was first ultrasonically cleaned for 15 minutes in sequence with 99.5% acetone, 99.7% isopropanol, and deionized water, and then pretreated with UV-O3 irradiation for 15 minutes.

[0099] EIL was prepared by spin-coating a precursor solution onto ITO glass at 3000 rpm for 60 s, followed by annealing at 220°C for 20 min. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 60 s. -4 Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) was deposited sequentially for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) was deposited for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) was deposited for HTL, MoO3 was deposited for HIL, and Al was deposited for anode, to obtain the first OLED device.

[0100] HIL was prepared by spin-coating a precursor solution onto ITO glass at 3200 rpm for 70 s, followed by annealing at 130°C for 10 min. Subsequently, the HIL-coated ITO glass was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm for 70 s. -4 Under vacuum, CBP for HTL, TAZ for EML, Bphen for ETL, LiF for EIL, and Al for cathode are sequentially vapor-deposited to obtain a second OLED device.

[0101] Comparative Example 1

[0102] (1) Preparation of urea solution

[0103] Dissolve 0.09g of urea in 15ml of deionized water, then place it in a high-pressure reactor and hydrothermally react at 185°C for 8 hours to prepare a pure urea solution.

[0104] (2) OLED device fabrication

[0105] The ITO glass was first ultrasonically cleaned for 15 minutes with 99.5% acetone, 99.7% isopropanol, and deionized water, respectively, and then pretreated with UV-O3 irradiation for 15 minutes.

[0106] EIL was prepared by spin-coating pure urea solution onto ITO glass at 3200 rpm for 70 s, followed by annealing at 200°C for 15 minutes. Subsequently, the ITO glass coated with EIL was transferred to a multi-source thermal evaporation system and annealed at 4 × 10⁻⁶ rpm. -4Under vacuum, 4,7-diphenyl-1,10-phenanthroline (Bphen) for ETL, 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) for EML, 4,4'-bis(9-carbazole)biphenyl (CBP) for HTL, MoO3 for HIL, and Al for anode were sequentially deposited to obtain an OLED device with the following structure:

[0107] Device EU: ITO / Urea / Bphen (25nm) / TAZ (35nm) / CBP (70nm) / MoO3 (5nm) / Al (110nm).

[0108] Experimental Example 1

[0109] (1) Experimental methods

[0110] The bipolar injection materials and OLED devices (device EU, device EN, device EC, device EZ, device HN, device HC, device HZ) prepared in Examples 1-3 and Comparative Example 1 were used.

[0111] The bipolar implanted material used in the device fabrication process was characterized using the following methods:

[0112] The surface morphology of the ITO glass substrate and the thin film prepared by spin coating annealing were examined using atomic force microscopy (AFM, Bruker MultiMode 8). The morphology and elemental composition of the thin films were characterized using scanning electron microscopy (SEM, FEI Quanta 450 FEG, ZEISS Sigma 360). The absorption properties of the thin films were measured using a UV-Vis spectrophotometer (PerkinElmer Lambda 365). The chemical state of the thin films was analyzed using X-ray photoelectron spectroscopy (XPS).

[0113] The following characterization methods were used to characterize and analyze the OLED devices:

[0114] The current density (J), voltage (V), irradiance, and electroluminescence (EL) spectrum of the OLED were characterized using a Keithley 2400 digital source meter, an Ocean Optics Maya-2000-Pro spectrometer, and data acquisition software.

[0115] (2) Results and Discussion

[0116] Figure 2AFM images of ITO anode, ITO / Urea, ITO / Urea+Ni, ITO / Urea+Co, and ITO / Urea+Zn are shown. Exposed ITO exhibits prominent grains, grain boundaries, and surface pinholes. The corresponding root mean square (RMS) roughness is 1.81 nm. Figure 2 (a) . Urea was coated onto ITO to fill some grain boundaries and surface pinholes, forming a uniform thin film, and the RMS roughness was reduced to 1.31 nm. Figure 2 (b)]. The ITO films coated with Urea+Ni, Urea+Co, and Urea+Zn all exhibited smooth surfaces, with RMS values ​​of 1.33 nm, 1.32 nm, and 1.17 nm, respectively. Figure 2 [(c), 2(d), 2(e)]. These phenomena indicate that Urea and transition metal doping can form high-quality thin films.

[0117] Figure 3 The images show SEM images of Urea+Ni, Urea+Co, and Urea+Zn, along with their corresponding elemental EDS diagrams. The images demonstrate that Urea+Ni, Urea+Co, and Urea+Zn are all granular, and their EDS results indicate that C, N, O, Ni, Co, and Zn elements coexist and are uniformly distributed on their respective films.

[0118] XPS measurement results for Urea, Urea+Ni, Urea+Co, and Urea+Zn thin films are as follows: Figure 4 As shown. C1s spectra of Urea, Urea+Ni, Urea+Co and Urea+Zn thin films [ Figure 4 (a) Both show C=C bonds (284.8 eV), C=O bonds (289 eV), and O1s spectra. Figure 4 (b) C=O bonds (533 eV) are present in both N1s spectra. Figure 4 (c) All films exhibit C-NH2 bonds (400.5 eV). This indicates that Urea, Urea+Ni, Urea+Co, and Urea+Zn films all contain urea. In the C1s spectrum, the C=O bond in urea (CO(NH2)2) has a lone pair of electrons. When Zn 2+ Co 2+ and Ni 2+ In the presence of ions, the lone pair of electrons on the C=O bond is partially transferred to Zn. 2+ (Co) 2+ or Ni 2+On the urea molecule, O→Zn (O→Co or O→Ni) coordination bonds are formed. This leads to a stronger attraction between the nucleus and the outer shell electrons, correspondingly increasing the binding energy. In the Urea+Zn system, the binding energy of C=O increases by about 0.6 eV. The Urea+Ni and Urea+Co systems exhibit a larger shift (about 1.3 eV), indicating stronger O→Ni and O→Co coordination interactions. This gives Urea+Ni and Urea+Co superior electron / hole injection capabilities compared to Urea+Zn. In the N1s spectrum, after coordination interactions, conjugated electron rearrangement occurs within the urea molecule. That is, the lone pair electrons on the N atom diffused from the amino group (-NH2) move towards C=O through the CNC=O conjugated system, resulting in a decrease in the C-NH2 binding energy. Compared to pure Urea, the Urea+Ni system exhibits a shift of about 0.3 eV, while the Urea+Co and Urea+Zn systems do not show a significant shift. This further demonstrates that Urea+Ni possesses the strongest electron / hole injection capability, resulting in superior OLED performance.

[0119] Figure 4 (d), 4(e), and 4(f) represent the 2P values ​​corresponding to Urea+Ni, Urea+Co, and Urea+Zn films, respectively. 3 / 2 and 2P 1 / 2 The presence of satellite peaks indicates that Ni, Co, and Zn were successfully doped in their oxidized states.

[0120] Figure 5 The UV-Vis absorption spectra of Urea, Urea+Ni, Urea+Co, and Urea+Zn films are shown. The results indicate that the films exhibit low light absorption in the emission wavelength region of the device, with no obvious absorption peaks. This low absorption characteristic suggests that Urea, Urea+Ni, Urea+Co, and Urea+Zn have minimal impact on the light output performance of UV OLEDs.

[0121] Figure 6 The JVR, EQE, and EL spectra of OLEDs using Urea, Urea+Ni, Urea+Co, and Urea+Zn as electron injection layers (EILs) are shown. Table 1 lists some typical performance parameters. The incorporation of Ni, Co, and Zn significantly improves the EQE of the OLED. At a current density of 111 mA / cm²... 2 At that time, the maximum external quantum efficiency of device EN reached 1.19%, and the current density was 135.2 mA / cm². 2 At that time, the maximum external quantum efficiency of the device EC reached 1.03%, and the current density was 154.4 mA / cm². 2 At that time, the maximum external quantum efficiency of the device EZ reached 0.81%. Figure 6(c)], compared with pure urea device EU (at 61.9 mA / cm 2 Compared to 0.67% at the time, the EQE was significantly improved. The material's excellent electron injection capability is attributed to the presence of the electron-donating group -NH2. With the incorporation of transition metals, its EQE improvement is due to the fact that the doping energy of transition metals can effectively promote electron injection / transport capability and balance the hole-electron ratio in the emitting layer. The electroluminescence spectrum of the fabricated OLED device shows the typical ultraviolet emission of TAZ molecules, producing an electroluminescence peak of 381-382 nm and a full width at half maximum (FWHM) of 44-45 nm. Figure 7 The EL spectra of the device under driving voltages of 8-12V are presented, and it is observed that the spectra are stable and independent of the driving voltage.

[0122] Table 1. Typical performance parameters of device EU, device EN, device EC and device EZ

[0123]

[0124] Figure 8 The JVR, EQE, and EL spectra of OLEDs using Urea+Ni, Urea+Co, and Urea+Zn as hole injection layers (HILs) are shown. Table 2 lists some typical performance parameters. (The last sentence appears to be incomplete and possibly refers to a different topic.) 2 At that time, the maximum external quantum efficiency of the HN device reached 1.73%, and the current density was 6.1 mA / cm². 2 At that time, the maximum external quantum efficiency of the device HC reached 0.85%, and the current density was 0.5 mA / cm². 2 At that time, the maximum external quantum efficiency of the device reached 0.35% at HZ. Figure 8 (c) The electroluminescence spectrum of the prepared OLED device shows typical ultraviolet emission of TAZ molecules, producing an electroluminescence peak of 376-377 nm and a full width at half maximum (FWHM) of 45-47 nm. Figure 9 The EL spectra of the device under driving voltages of 8-12V are presented, and the spectra are observed to be stable and independent of the driving voltage. Urea+Ni, Urea+Co, and Urea+Zn materials exhibit hole injection capability because the conjugated rearrangement of the transition metal matrix unit with a wide valence band and urea molecules results in good hole injection capability.

[0125] Table 2. Some typical performance parameters of devices HN, HC and HZ

[0126]

[0127] Experiment Example 2

[0128] (1) Experimental methods

[0129] To further analyze the differences in electron injection and hole injection performance of different thin films, EOD devices (i.e., electron-single-carrier devices) with Urea, Urea+Ni, Urea+Co, and Urea+Zn as EILs, and HOD devices (i.e., hole-single-carrier devices) with Urea+Ni, Urea+Co, and Urea+Zn as HILs were fabricated according to the process parameters of Examples 1-3 and Comparative Example 1, respectively. Current-voltage (IV), impedance-voltage (ZV), and phase angle-voltage (Φ-V) tests were performed using an Agilent 4294A precision impedance analyzer. The structures of the EOD and HOD devices are as follows:

[0130] EOD devices:

[0131] Device E1: ITO / Urea / Bphen (45nm) / MoO3 (5nm) / Al;

[0132] Device E2: ITO / Urea+Ni / Bphen (45nm) / MoO3 (5nm) / Al;

[0133] Device E3: ITO / Urea+Co / Bphen (45nm) / MoO3 (5nm) / Al;

[0134] Device E4: ITO / Urea+Zn / Bphen(45nm) / MoO3(5nm) / Al.

[0135] HOD devices:

[0136] Device H1: ITO / Urea+Ni / CBP(110nm) / Al;

[0137] Device H2: ITO / Urea+Co / CBP(110nm) / Al;

[0138] Device H3: ITO / Urea+Zn / CBP(110nm) / Al.

[0139] (2) Results and Discussion

[0140] like Figure 10 As shown in the IV curves in (a), the currents of devices E1-E4 all increase with increasing voltage, indicating that carrier transport in devices E1-E4 is stable. Figure 10 As can be seen from the impedance spectra of the ZV and Φ-V curves shown in (b) and (c), at low voltages less than 2 V, all EOD devices exhibit an impedance of approximately 0.6 × 10⁻⁶. 5The high impedance of Ω and the negative phase angle of approximately -90° indicate that the device is in an insulating state. As the voltage increases, the impedance of devices E1-E4 all decrease, gradually transitioning from a high-impedance insulating state to a semiconductor state, and the phase angle begins to change from -90° to 0°. This indicates that EOD transitions from an insulating state to a semiconducting or conductive state, and also shows that the prepared thin films all have good electron injection capabilities.

[0141] Similarly, such as Figure 11 The IV curves in (a) show that the currents of devices H1-H3 all increase with increasing voltage, indicating that carrier transport in devices H1-H3 is stable. From... Figure 11 As can be seen from the impedance spectra of the ZV and Φ-V curves shown in (b) and 11(c), at low voltages less than 1 V, all HOD devices exhibit high impedance and a negative phase angle of approximately -90°. As the voltage increases, the devices gradually transition to a low-resistance state, and the phase angle gradually changes towards 0°. This indicates that the devices change from an insulating state to a semiconducting or conductive state, and also indicates that the prepared thin films all have good hole injection capabilities.

[0142] In summary, the surface morphology and electron / hole injection capabilities of spin-coated Urea, Urea+Ni, Urea+Co, and Urea+Zn were systematically characterized using AFM, SEM, XPS, IV, and impedance spectroscopy. With TAZ as the emitting layer and Urea+Ni as the composite hole injection layer, the OLED achieved a maximum EQE of 1.73% and an irradiance of 4.3 mW / cm². 2 When used as a composite electron injection layer, the maximum EQE is 1.19%, and the irradiance is 12.9 mW / cm². 2 The EL spectrum is stable. Experimental results show that transition metals nickel, cobalt, and zinc provide a new feasible scheme for constructing bipolar OLED carrier injection layers.

Claims

1. A method for preparing a bipolar carrier injection material, characterized in that: The material is generated by hydrothermal reaction of urea and transition metal salt; the transition metal salt contains at least one of nickel, cobalt, and zinc; the hydrothermal reaction is carried out at a temperature of 175-185°C for 8-12 hours.

2. The preparation method according to claim 1, characterized in that: The transition metal salt is selected from at least one of nickel nitrate, cobalt nitrate, and zinc nitrate.

3. The preparation method according to claim 1, characterized in that: The molar ratio of urea to transition metal salt is 1:(0.8–1.5).

4. The preparation method according to claim 1, characterized in that: It also includes purification, the process of which is to cool the solution after the hydrothermal reaction to room temperature and centrifuge to obtain the supernatant.

5. The preparation method according to claim 4, characterized in that: The centrifugation conditions are: 10000–15000 rpm, 5–10 minutes.

6. The preparation method according to claim 4, characterized in that: It also includes film coating, the process of which is as follows: spin coating the supernatant onto the substrate, annealing, and obtaining a thin film.

7. The preparation method according to claim 6, characterized in that: The spin coating is performed at a speed of 3000–3500 rpm for 60–80 seconds; the annealing temperature is 130–220°C for 10–20 minutes.

8. A bipolar carrier injection material prepared by the preparation method according to any one of claims 1–7.

9. The application of the bipolar carrier injection material as described in claim 8 in the fabrication of organic light-emitting diode devices.

10. An organic light-emitting diode (OLED) device, comprising an anode, a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, an electron injection layer, and a cathode, characterized in that: The hole injection layer and / or electron injection layer are formed by coating the bipolar carrier injection material as described in claim 8.

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